The American screen has long been a smoky place, at least since 1942's Now, Voyager, in which Bette Davis and Paul Henreid showed how to make and seal a romantic deal over a pair of cigarettes that were smoldering as much as the stars. Today cigarettes are more common onscreen than at any other time since midcentury: 75% of all Hollywood films--including 36% of those rated G or PG--show tobacco use, according to a 2006 survey by the University of California, San Francisco.

Audiences, especially kids, are taking notice. Two recent studies, published in Lancet and Pediatrics, have found that among...